6.2 Drainage Sumps, Sewage Ejectors & Backwater Valves

6.2 Drainage Sumps, Sewage Ejectors & Backwater Valves

Quick Answer: Under IPC Sections 712 and 715, plumbing fixtures installed below the gravity sewer invert must discharge into an airtight, gas-tight, gasket-sealed sump basin. Sewage ejector pumps handling sanitary waste must be non-clog solids-handling centrifugal units capable of passing a 2-inch solid sphere through a minimum 2-inch discharge pipe (or grinder pumps discharging through minimum 1-1/4 inches). The ejector discharge line must feature an accessible check valve positioned between the pump and a fullway gate/ball shutoff valve. Sump pits are vented under Chapter 9, with sump vent size read from Table 916.5.1 by pump discharge capacity and developed length; Indiana's amended Section 917.8 bars air admittance valves on sump pits. Furthermore, Section 715.1 requires a backwater valve where fixtures sit on a floor whose finished floor elevation is below the elevation of the manhole cover of the next upstream public sewer manhole, and fixtures above that elevation must not discharge through it.

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Subgrade Drainage & Hydraulic Principles (IPC 712.1)

In standard plumbing systems, gravity is the driving force conveying wastewater from fixture outlets down into the municipal sewer or private septic tank. However, in modern residential and commercial construction, basements, underground parking facilities, and subgrade storage levels frequently place plumbing fixtures below the physical elevation of the gravity street sewer.

Under IPC Section 712.1, drainage piping that cannot discharge into the building sewer by gravity flow must discharge into an approved, tightly covered drainage sump or sewage ejector pit located at the lowest subgrade level. From this basin, the sewage must be lifted mechanically and discharged into the gravity building drain or sewer.

+-------------------------------------------------------------------------+
|                   SUBGRADE LIFT SYSTEM ARCHITECTURE                     |
+-------------------------------------------------------------------------+
|                                                                         |
|   GRAVITY BUILDING DRAIN (Above Grade) <===== [ DISCHARGE LINE ]        |
|                                                      ^                  |
|                                                      |                  |
|                                               [Gate Valve]              |
|                                               [Check Valve]             |
|                                                      |                  |
|   +------------------------------------+             |                  |
|   | AIRTIGHT GASKETED SUMP COVER       |             |                  |
|   |   (Vent Pipe)       (Discharge)    |             |                  |
|   +-------||-----------------||--------+             |                  |
|           ||                 ||                      |                  |
|           ||                 ||                      |                  |
|     [AIRTIGHT BASIN]         ||                      |                  |
|     (Poly / Fiberglass)      ||                      |                  |
|                              ||                      |                  |
|     Subgrade Inflow ===>     ||                      |                  |
|     (Gravity Drain)          ||                      |                  |
|                              ||======================+                  |
|                        [SEWAGE EJECTOR]                                 |
|                    (Passes 2" Solid Sphere)                             |
+-------------------------------------------------------------------------+

Clear Water vs. Sanitary Sewage

Plumbing codes enforce an absolute structural and hydraulic separation between two distinct classes of subgrade drainage:

1. Subsoil / Groundwater Sumps: Pits collecting clear foundation groundwater, footing drain discharge, or basement floor rainwater must never discharge into the sanitary drainage system. They must be pumped to an approved storm sewer, drywell, or surface swale.
2. Sewage Ejector Sumps: Pits collecting fecal discharge, graywater from basement laundry, lavatories, or subgrade floor drains must discharge exclusively into the sanitary sewer. Cross-connecting clear footing water into a sanitary sewage ejector overloads municipal treatment plants and violates both state environmental laws and local plumbing regulations.

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Sewage Ejector Pump Engineering & Solids Handling (IPC 712.3)

A standard sump pump designed for clear water removal utilizes a tight, high-speed closed impeller that instantly jams when exposed to paper, textiles, or human waste. Sanitary drainage systems require specialized sewage ejectors designed to handle raw waste.

Solids-Handling Centrifugal Pumps

The solids rule lives in Section 712.4.2 (Capacity), not 712.3.3. It reads: pumps or ejectors that receive the discharge of water closets shall be capable of handling spherical solids up to and including 2 inches (51 mm); other pumps or ejectors shall handle spherical solids up to and including 1 inch (25.4 mm). (Section 712.3.3 simply says discharge piping shall meet the requirements of Section 712.2.)

• Impeller Design: These pumps incorporate vortex or semi-open recessed impellers that generate a powerful centrifugal vortex within the pump volute. Solids are drawn into the suction opening and swept through the casing without directly impacting narrow impeller vanes.
• Discharge Sizing: Table 712.4.2 (Minimum Capacity of Sewage Pump or Sewage Ejector) ties the discharge pipe diameter to a minimum pump capacity: 2 inches → 21 gpm, 2-1/2 inches → 30 gpm, 3 inches → 46 gpm. The table starts at 2 inches, which is why a conventional solids-handling ejector discharges through a line of not less than 2 inches nominal.

Macerating & Sewage Grinder Pumps

The exceptions to Section 712.4.2 permit grinder pumps and macerating toilet assemblies as an alternative to 2-inch solids handling. Section 712.4.1 separately requires macerating toilet systems to comply with CSA B45.9 or ASME A112.3.4 and to be installed per the manufacturer's instructions.

• Operating Mechanism: Grinder pumps incorporate high-speed hardened stainless steel cutter rings and rotating shredder bars spinning at 3,450 RPM directly beneath the suction port. Fecal matter, sanitary tissues, and solids are pulverized into a fine slurry before reaching the impeller.
• Discharge Sizing: Because solids are mechanically liquefied, the exceptions to 712.4.2 allow smaller openings: grinder pumps or grinder ejectors receiving the discharge of water closets shall have a minimum discharge opening of 1.25 inches (32 mm), and macerating toilet assemblies serving single water closets shall have a minimum discharge opening of 0.75 inch (19 mm).

Duplex Pump Arrangements in Public Buildings

In single-family residences, a simplex (single pump) ejector is acceptable. The 2006 IPC does not mandate duplex pumping anywhere in Section 712 — that requirement comes from the engineer's design, the owner's standard, or a local ordinance. Where reliability matters (commercial buildings, healthcare, multi-family), a duplex pump system is the normal design:

• Two identical pumps are installed in a common sump basin.
• An alternating control panel alternates the lead pump on each cycle to equalize motor wear.
• If the inflow exceeds the capacity of one pump or if the lead pump fails, a high-water float switch activates both pumps simultaneously and energizes an audible and visual high-water alarm.

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Sump Basin Construction, Gaskets & Gas-Tight Sealing

A sewage ejector pit collects raw sanitary waste, human pathogens, and hazardous sewer gases, including toxic hydrogen sulfide ($H_2S$) and explosive methane ($CH_4$). The basin must therefore function as a sealed pressure vessel against atmospheric gas leakage.

+-------------------------------------------------------------------------+
|                    SEWAGE BASIN CONSTRUCTION CRITERIA                   |
+-------------------------------------------------------------------------+
| Basin Wall Materials    | Fiberglass, polyethylene, coated iron, concrete|
| Cover Construction      | Bolted rigid lid with elastomeric gasket seal  |
| Electrical Penetrations | Gas-tight neoprene compression cord grommets   |
| Venting Requirement     | Vented per Ch. 9; IN 917.8 bars AAVs on sump pits|
| Minimum Water Depth     | Engineered to submerge pump volute & cooling   |
+-------------------------------------------------------------------------+

• Basin Material: Basins must be fabricated from non-corrosive, impervious materials such as heavy rotational-molded polyethylene, structural fiberglass-reinforced resin, cast iron, or precast waterproof concrete.
• Code text (712.3.2 Sump Pit): "The sump pit shall be not less than 18 inches (457 mm) in diameter and 24 inches (610 mm) deep, unless otherwise approved. The pit shall be accessible and located such that all drainage flows into the pit by gravity. The sump pit shall be constructed of tile, concrete, steel, plastic or other approved materials. The pit bottom shall be solid and provide permanent support for the pump. The sump pit shall be fitted with a gas-tight removable cover adequate to support anticipated loads in the area of use. The sump pit shall be vented in accordance with Chapter 9."
• Gas-Tight Lid: The basin lid must be a heavy rigid cover bolted down around its perimeter with non-corrosive machine screws. A continuous, high-density closed-cell elastomeric rubber gasket must be sandwiched between the lid and the basin rim flange to guarantee an airtight, gas-tight, and water-tight seal.
• Cord Grips: Electric power cords for the pump motor and liquid level float switches must penetrate the lid through specialized threaded neoprene compression glands (cord grips) that seal hermetically around the cable jacket.

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Sump Vent Sizing & Airflow Dynamics (IPC Section 916.5.1 and Table 916.5.1)

Every sewage ejector basin must be provided with an atmospheric vent pipe. As water rushes into the basin from gravity fixtures, air inside the pit is rapidly displaced; conversely, when the ejector pump fires at high flow rates (often 30 to 80 gallons per minute), it creates a severe internal vacuum.

Prohibition of Air Admittance Valves (AAVs)

Under no circumstances may an Air Admittance Valve (AAV) be installed on a sewage ejector sump basin. An AAV is a one-way mechanical valve designed solely to admit ambient air under negative pressure. When wastewater enters the sump basin, the displaced air creates positive pressure inside the pit. An AAV cannot relieve positive pressure; the trapped air will either blow out fixture trap seals on the subgrade line or cause sewer gases to rupture the basin gasket seals. Sump vents must terminate outdoors through the roof or connect to the building vent system above the fixture flood level rim.

Where Sump Vent Sizing Actually Lives

Sump vents are sized in Chapter 9, not Chapter 7. Section 916.5 (Sump Vents) sends you to 916.5.1 for pumps and ejectors other than pneumatic, which states that drainage piping below sewer level shall be vented in a similar manner to a gravity system and that building sump vent sizes shall be determined in accordance with Table 916.5.1. (Table 712.4.2 is a different table entirely — it gives the minimum capacity of a sewage pump or ejector by discharge pipe diameter: 2 inches → 21 gpm, 2-1/2 inches → 30 gpm, 3 inches → 46 gpm.)

Table 916.5.1 — Size and Length of Sump Vents (maximum developed length of vent, in feet):

Reading the table: the developed length used for lookup includes an allowance for entrance losses and fitting friction; where a more precise value is unavailable, the code directs you to assume an allowance of 50 percent of the developed length. "No limit" in the table means actual values greater than 500 feet, and the "10" entries flagged in the code mean less than 10 feet.

Pneumatic ejectors (916.5.2): the air pressure relief pipe from a pneumatic sewage ejector connects to an independent vent stack terminating as required for vent extensions through the roof, sized to relieve ejector air pressure to atmosphere but not less than 1-1/4 inches (32 mm).

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Discharge Piping & Mandatory Valve Configuration (IPC 712.2)

The pressurized discharge line from a sewage ejector must be engineered with specific valves and fittings arranged in a precise sequence to prevent liquid backflow and allow pump maintenance.

                      DISCHARGE PIPING VALVE STACK

     ==================== GRAVITY MAIN (Building Drain)
              ^
              |
        [ GATE VALVE ]  <-- Fullway shutoff valve (isolate system)
              |
        [CHECK VALVE]   <-- Swing check valve (prevent backflow)
              |
        [DISCHARGE PIPE] (Minimum 2" for solids pump)
              |
       [EJECTOR BASIN LID]
              |
        [EJECTOR PUMP]

The Mandatory Valve Sequence

The valve rule is Section 712.2 (Valves Required), not 712.3.2. It reads: "A check valve and a full open valve located on the discharge side of the check valve shall be installed in the pump or ejector discharge piping between the pump or ejector and the gravity drainage system. Access shall be provided to such valves. Such valves shall be located above the sump cover required by Section 712.1 or, where the discharge pipe from the ejector is below grade, the valves shall be accessibly located outside the sump below grade in an access pit with a removable access cover."

• Relative Placement: the check valve sits first, with the full open valve on its discharge side — that is, between the pump and the shutoff valve.
• Why this Sequence Matters: When the ejector pump shuts off, the vertical column of water inside the discharge pipe rests against the check valve flapper. If the pump fails or requires maintenance, a plumber must be able to service the system. By closing the fullway gate valve (located downstream, above the check valve), the plumber isolates the entire building drainage head. The check valve, pump union, and pump assembly can then be disconnected without gallons of raw sewage dumping out of the overhead drainage line into the basement.
• Valve Specifications:
  • Check Valve: Must be a horizontal swing check valve or spring-loaded flapper check valve with full-flow waterway. Rubber flapper weighted valves are preferred to prevent "water hammer" slam when the pump stops.
  • Shutoff Valve: Must be a fullway valve (a gate valve or full-port ball valve) that provides an unobstructed bore equal to the inside pipe diameter when open. Globe valves are strictly forbidden because their internal tortuous path causes solids to clog instantly.

Gravity Connection Criteria

Section 712.3.5 (Ejector connection) governs: "Pumps connected to the drainage system shall connect to the building sewer or shall connect to a wye fitting in the building drain a minimum of 10 feet (3,048 mm) from the base of any soil stack, waste stack or fixture drain." The 10-foot standoff keeps the pumped surge away from the hydraulic jump at a stack base, where it would otherwise siphon or blow nearby trap seals.

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Backwater Valves: Municipal Surcharge Protection (IPC Section 715)

During extreme rainstorms, municipal combined sewers and sanitary mains frequently become overloaded with stormwater infiltration. When the main surcharges, the hydraulic grade line rises, forcing raw municipal sewage backward into private building drains.

                 MUNICIPAL SEWER SURCHARGE SCENARIO

   STREET GRADE                  UPSTREAM MANHOLE COVER
  ===================================== [===] =========================
                                          |  ^ Surcharge Hydraulic Level
                                          |  | forces sewage back
                                          |  v
   +-----------------------+              |
   | BASEMENT FIXTURES     |              |
   | Flood rim BELOW cover |              |
   |   [Toilet / Drain]    |              |
   +----------||-----------+              |
              ||                          |
              v                           |
       [BACKWATER VALVE]                  |
       (Flapper closes to                 |
       block city backup)                 |
              ||                          |
              ============================+===> MUNICIPAL MAIN SEWER

The Code Trigger for Backwater Valves (IPC 715.1)

Section 715.1 reads: "Where plumbing fixtures are installed on a floor with a finished floor elevation below the elevation of the manhole cover of the next upstream manhole in the public sewer, such fixtures shall be protected by a backwater valve installed in the building drain, branch of the building drain or horizontal branch serving such fixtures. Plumbing fixtures installed on a floor with a finished floor elevation above the elevation of the manhole cover of the next upstream manhole in the public sewer shall not discharge through a backwater valve." Note the measuring point: finished floor elevation, not flood level rim.

The Golden Rule of Backwater Valve Installation

• Only Subgrade Fixtures: Only fixtures on a floor whose finished floor elevation is below the upstream manhole cover may discharge through the backwater valve.
• Upper Floor Bypass: Fixtures on a floor whose finished floor elevation is above the upstream manhole cover shall not discharge through the backwater valve — that is code text, not just good practice.
• The Catastrophic Failure Scenario: If upper-floor fixtures are mistakenly routed through the backwater valve, consider what occurs during a municipal surcharge: the street main backs up, closing the backwater flapper. If residents on the second floor continue flushing toilets, taking showers, and running sinks, their gravity wastewater hits the closed flapper, finds no exit, and backs up directly out of the basement floor drains and toilets! Routing upper fixtures around the backwater valve ensures they drain continuously to the street while basement fixtures remain protected.

Valve Construction and Accessibility

Backwater valves must be gate-type or swing-flapper type with corrosion-resistant brass, PVC, or stainless steel internals and a resilient neoprene seat. Under IPC Section 715.5, backwater valves must be permanently accessible for maintenance, clearing, and flapper replacement through a finished floor access cover, pit, or extension housing.

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Summary Reference Tables

Table 1: Subgrade Drainage & Valve Comparison

Table 2: Sewage Ejector Discharge Pipe Minimum Dimensions